1. Field of the Invention
The present invention relates generally to a magnetoresistive element having an change coupling film utilizing an exchanging coupling between an antiferromagnetic film and a ferromagnetic film, a magnetic head using the magnetoresistive element, and a magnetic disk drive using the magnetic head.
2. Description of the Background
As a read head in a high density magnetic recording, a magnetic head using a magnetoresistive element has been studied. At present, an 80 at % Ni-20 at % Fe (coummn name: permalloy) alloy thin-film is used as a material of the magnetoresistive element. In recent years, as materials substituted for this, artificial lattice films and spin valve films, such as (Co/Cu)n, which have a giant magnetoresistance effect, are widely noticed.
Since the magnetoresistance effect film of permalloy has magnetic domains, the Barkhausen noises resulting from this are much of a problem for practical use. Therefore, various methods for causing a magnetoresistance efts film to have a single magnetic domain are studied. As one of the methods, there is used a method for controlling the magnetic domains of a magnetoresistance effect film in a specific direction using an exchanging coupling between a magnetoresistance effect film, which is a ferromagnetic film, and an antiferromagnetic film. As the antiferromagnetic material, .gamma.-FeMn alloy is well known (e.g., U.S. Pat. No. 4,103,315 and U.S. Pat. No. 5,015,147). This magnetoresistance effect is called anisotropic magnetoresistance effect.
Moreover, in recent years, the art utilizing an exchanging coupling between an antiferromagnetic film and a ferromagnetic film is widely used in order to pin the magnetization of a magnetic film of a spin valve film. Also for this purpose, .gamma.-FeMn alloy is widely used as the antiferromagnetic film.
However, the .gamma.-FeMn alloy has the problem of corrosion resistance, particularly corrosion due to water, so that there is a problem in that the exchange coupling field to a magnetoresistance effect film as a magnetoresistive element is deteriorated by corrosion at a processing step or corrosion due to water in atmosphere as time elapses.
In addition, the pentium processor recently incorporated in a machine having an accelerated throughput has a very large heating value, so that the temperature in an HDD also rises to about 150.degree. C. during operation. Therefore, an exchange coupling field of 200 Oe or more at 150.degree. C. is required in view of reliability. In order to obtain an exchange coupling field of 200 Oe or more at 150.degree. C., it is desired that the exchange coupling field at room temperature is not only high, but the temperature characteristic of the exchange coupling field is also good. Moreover, it is required that the blocking temperature, at which the exchanging coupling between the ferromagnetic film and the antiferromagnetic film disappears, should be as higher as possible.
However, the blocking temperature of the .gamma.-FeMn alloy is 170.degree. C. or lower. In addition, the temperature characteristic of the exchange coupling field is very bad. Therefore, the exchange coupling field is not sufficient at 100.degree. C., so that there is a problem in that there is no long-term reliability.
In addition, U.S. Pat. No. 4,103,315 discloses the use of oxides, such as NiO. Moreover, U.S. Pat. No. 5,315,468 discloses that if an antiferromagnetic film is formed of .theta.-Mn alloy, such as NiMn alloy, which has a face centered tetragonal crystal structure, the exchange coupling field between the antiferromagnetic film and the ferromagnetic film does not deteriorate even in a high temperature range.
Moreover, the inventor has proposed an antiferromagnetic film of IrMn having a face centered cubic crystal structure, which has excellent characteristics. In addition, U.S. Pat. No. 5,315,468 discloses that other .gamma.-Mn alloys, such as MnPt and MnRh, are used as the antiferromagnetic films of the same crystal structure.
However, these antiferromagnetic films are formed of Mn alloy which is difficult to prepare a high density target, so that it is difficult to manage the quality of the film. In addition, when the antiferromagnetic films are laminated on the top of a ferromagnetic film or a spin valve film, the ferromagnetic film or the spin valve film supports an under layer for the antiferromagnetic film, so that the antiferromagnetic film crystal-grows so as to obtain good exchanging coupling characteristics. However, when the antiferromagnetic underlies the ferroelectric film, there is a problem of the selection of the under layer that promotes a crystal growth.
On the other hand, a magnetic storage having a magnetic head, which uses a magnetoresistive element having a dual spin valve film and a magnetoresistive element having a dual spin valve film, is also widely used.
Conventionally, the readout of information recorded in a magnetic recording medium is carried out by a method for moving a read head comprising a magnetic core, onto which a coil is wound and which has a magnetic gap, with respect to a recording medium, and sensing a magnetic field through the magnetic gap at that time to detect a voltage induced in the coil. On the other hand, with the increase of the magnetic recording density, a magnetic head (MR head) utilizing the magnetoresistance effect (MR effect), such as NiFe alloy film, is widely used at present, since it is able to more sensitively recorded information out of a magnetic recording medium.
Recently, in order to more increase the magnetic recording density, high sensitive magnetoresistive elements using a higher sensitive giant magnetoresistance effect (GMR) than those of the MR heads, i.e., GMR elements, are developed. The promising of the GMR elements is a structure called a spin valve structure. This comprises a non-magnetic metal layer sandwiched between two ferromagnetic metal layers. In this structure, when the direction of magnetization of one of the magnetic layers (the free layer) varies with respect to another layer, the magnetization of which is fixed, by a magnetic field from a recording medium, it is possible to obtain information in the magnetic field of the recording medium as a large variation in value of resistance.
In order to obtain a high output using such a spin valve structure, various structures have been proposed. One of them is a structure called a dual spin valve. In this structure, a free layer is arranged between two magnetization fixed layers, the magnetizations of which are fixed in the same direction, via a non-magnetic metal layer. According to this dual spin valve structure, there is an advantage in that it is possible to obtain a higher output than that of a conventional spin valve film having a single magnetization fixed layer.
However, although the above described spin valve structure can obtain a high output, there are problems in that there are some cases where the pinned layer is inverted by the electrostatic discharge (ESD) so that the output can be obtained, and that it is difficult to modify this to obtain the output again. In addition, it is difficult to set the bias point of the element since a great bias magnetic field in the spin valve.
That is, there are some cases where the pinned layer is inverted by the electrostatic discharge (ESD) in the conventional spin valve element. In order to modify the inversion of the pinned layer, there is proposed a circuit for passing a current through the element to add its galvano magnetic field to the pinned layer. However, in the case of the conventional dual spin valve structure, if the current flows through the element to add its galvano magnetic field to the magnetization fixed layer, magnetic fields are applied to two pinned layers in opposite directions to each other, so that the two pinned layers are fixed in opposite directions to each other. However, in the dual spin valve structure, it is not possible to obtain the output due to the variation in magnetic resistance unless the direction of the magnetization of the pinned layer is the same direction. Therefore, there is a problem in that the method for modifying the inversion of the pinned layer for use in the conventional spin valve element can not be applied to the conventional dual spin valve structure.
On the other hand, in the dual spin valve structure, if the pinned layer is set so as to obtain a high output, a large bias magnetic field is produced from the pinned layer to the free layer, so that it is difficult to get the equivalently outputs for the positive and negative components of the magnetic field. The reason for this is that since two pinned layers exist, if the pinned layers are intended to be design so as to obtain a high output, the total value of products obtained by multiplying the saturation magnetization Ms by the thickness t increases, so that the bias magnetic field to the free layer increases. If the total value of Ms.multidot.t of the two pinned layers exceeds the value of Ms.multidot.t of the free layer, the magnetizations of the free layer and pinned layer are completely parallel to each other in opposite directions to each other due to the magnetostatic coupling. In this case, in one of the positive and negative directions of the magnetic field, it is not possible to obtain the variation in output even if the magnetic field varies.
On the other hand, in order to increase the magnetic recording density of the magnetoresistance effect head in the magnetic disk drive, the MR head using the magnetores distance effect (MR effect) element for the read head part plays an important part. In order to achieve a higher density hereafter, it is required to provide an MR head using the giant magnetoresistance effect (GMR effect) element, which greatly increases the sensitivity of the MR effect element, for the read head.
As shown in FIG. 1, in a conventional shielding MR head using the GMR effect, a lower magnetic shield layer 2 of a soft magnetic film, such as permalloy, is formed on a substrate 1 of, e.g., Al.sub.2 O.sub.3.multidot.TiC. An MR film 4 (spin valve) is arranged on this magnetic shield layer via an insulator film 3 constituting a read magnetic gap. in The MR film 4 includes a so-called free layer 15, which rotates the magnetization in accordance with a signal magnetic field, an intermediate layer 14, a pinned layer 16, and an antiferromagnetic layer 17. The magnetization of the pinned layer 16 is pinned by the antiferromagnetic layer 17. In addition, in order to cause the magnetization of the free layer 15 to be a single magnetic domain, a pair of longitudinal bias films 5 of Copt or the like for producing a bias magnetic field, and a pair of leads 6 are arranged on both sides of the MR film 4 to form a magnetoresistive element (which will be hereinafter referred to as an "MR element") 7 of an abutted junction system. On the MR element 7, an insulator film 8 constituting a read magnetic gap, and an upper magnetic shield 9 are arranged. In such a shielding MR head, the signal magnetic field is detected by, e.g., passing a sense current through the pair of leads 6 to measure the variation of resistance of the film due to the variation of the average magnetization direction of the MR film 4.
Conventionally, the MR film using the GMR element is formed of a spin valve film. The basic construction thereof comprises a free layer/a non-magnetic spacer layer/a pinned layer. In addition, an antiferromagnetic layer is laminated on the pinned layer, and the structure of the free layer/the non-magnetic spacer layer/pinned layer/the antiferromagnetic layer is formed. The magnetization of the pinned layer is pinned by the exchange coupling field from the antiferromagnetic layer.
In the MR film using the GMR film, in order to insure a linear response for the magnetization of the free layer, it is required that the magnetization of the free layer, when no magnetic field exists, should be substantially perpendicular to the magnetization of the pinned layer. An example of this heat treatment process 3is as follows. After the induced magnetic anisotropy is applied to the free layer and the magnetic shield layer at about 250.degree. C. while applying a magnetic field thereto, the direction of the magnetic field is rotated by 90.degree. to carry out cooling. After cooling, the magnetic field is applied again along the direction of the free layer to increase the temperature up to 150.degree. C. so as to carry out coolong. And the orthogonal alignment between the magnetization of the free layer and the magnetization of the pinned layer is realized. After the heat treatment, a bias magnetic field is applied to the free layer by a hard magnetic layer, to inhibit the production of Barkhausen noises.
However, in the MR film and MR head, which use the GMR element, if the blocking temperature (which will be hereinafter referred to as T.sub.B) of the antiferromagnetic layer is designed to be high in order to enhance the thermal stability of the pinned layer, it is required to carry out heat treatment at a high temperature to pin the magnetization of the pinned layer. As a result, it is insufficient to apply the induced magnetic anisotropy to the magnetization free end and magnetic shield layer although the thermal stability of the pinned layer is enhanced.
On the other hand, if the T.sub.B is set to be low, the induced magnetic anisotropy can be applied to the free layer and magnetic shield layer, but the thermal stability of the pinned layer is not sufficient. Therefore, the output of the head is deteriorated by the temperature rise during operation of the head, e.g., at 100.degree. C. or higher.
In addition, there is also a problem in that if the T.sub.B is low, the reverse of magnetization of the pinned layer is caused by an ESD (electrostatic discharge). This has an influence on the producing yield in the production of the element and the assembly of a disk drive. Thus, in the conventional construction, it is difficult to combine both the stabilities of the induced magnetic anisotoropy and the pinned layer.
On the other hand, there are known a GMR film having the construction of a pinned layer formed directly of a hard magnetic layer as a pinned layer, and a GMR film having the construction of a pinned layer is pinned by a hard magnetic layer. Thus, it is possible to apply the induced magnetic anisotropy to the free layer and magnetic shield layer by heat treatment to fix the magnetization of the pinned layer by polarization at room temperature. However, with such construction, the magnetic field out of the hard magnetic layer increases, so that it is difficult to set the bias point similar to the use of the antiferromagnetic layer. In addition, unless the coercive force of the hard magnetic layer increases, the pinned layer is moved by the signal field of the medium, so that a desired output is not obtained.
As described above, the thermal stability of the pinned layer of the GKR element greatly depends on the T.sub.B of the antiferromagnetic layer. If the T.sub.B is designed to be high, although the thermal stability of the pinned layer is enhanced, it is required to carry out the high-temperature heat treatment of the antiferromagnetic layer, so that it is difficult to apply the induced magnetic anisotropy to the shield layer and free layer by the heat treatment. on the other hand, if the T.sub.B is designed to be low, although it is easy to apply the induced magnetic anisotropy to the shield layer and free layer, the thermal stability of the pinned layer is insufficient, so that the output voltage of the head and the producing yield of the head are deteriorated.